1. Field of the Invention
This invention relates to a method of making a magnetic recording medium by applying a magnetic recording layer onto a strip-like non-magnetic substrate by a coating process, and an apparatus for carrying out the method. This invention particularly relates to a method of making a magnetic recording medium in which magnetic grains in the magnetic recording layer are randomly orientated, and an apparatus for carrying out the method.
2. Description of the Prior Art
In general, magnetic recording media such as flexible magnetic disks and magnetic sheets are made by continuously moving a strip-like non-magnetic substrate in the longitudinal direction thereof, and applying a magnetic coating composition onto the strip-like non-magnetic substrate by a known coating process such as top reverse-roll coating, bottom reverse-roll coating, doctor coating or gravure coating. The strip-like non-magnetic substrate is fabricated, for example, of polyethylene terephthalate, triacetyl cellulose, diacetyl cellulose, polyvinylidene chloride or polypropyrene. The magnetic coating composition comprises a binder dissolved in a solvent, and ferromagnetic fine grains such as .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-doped .gamma.-Fe.sub.2 O.sub.3, Co-doped Fe.sub.3 O.sub.4, and CrO.sub.2 dispersed therein. As the binder, there has heretofore been used a vinyl chloride-vinyl acetate copolymer, a vinyl chloride-acrylonitrile copolymer, an acrylic ester-acrylonitrile copolymer, an acrylic ester-vinylidene chloride copolymer, other copolymers with acrylic acid, an urethane elastomer, a nylon-silicone resin, nitrocellulose, polyvinyl chloride, a vinylidene chloride-acrylonitrile copolymer, a polyamide resin, a polyvinyl butyral, a cellulose derivative, a styrene-butadiene copolymer, a phenolic resin, an epoxy resin, a polyurethane, a urea resin, a melamine resin, a polyester resin, a chlorovinyl ether-acrylic ester copolymer, a methacrylate copolymer-diisocyanate blend polymer, an amino resin, or a synthetic rubber. Then, the coating film thus applied onto the strip-like non-magnetic substrate is dried and hardened, and the substrate is punched or cut to form the magnetic recording medium. However, when the ferromagnetic fine grains (hereinafter referred to as magnetic grains) are orientated in a specific direction during manufacturing of the magnetic recording medium and anisotropy is generated in the magnetic recording medium, anisotropy also arises in the magnetic characteristics and the electromagnetic characteristics in various directions.
For example, in the case where the magnetic recording medium is a magnetic disk, when the magnetic grains are orientated in the coating direction of the magnetic coating composition, i.e. in the movement direction of the non-magnetic substrate, the level of reproduction output signal in the coating direction becomes higher than those in the other directions and, as a result, the level of reproduction output signal detected from the magnetic disk changes as the magnetic disk rotates. This phenomenon is generally called modulation.
Accordingly, when magnetic disks or the like are made by the coating process, an attempt has heretofore been made to physically eliminate orientation of the magnetic grains or to shield the magnetic field so that it does not act on the magnetic grains. However, there still arises the problem that the magnetic grains exhibit linear directivity due to flow orientation during application of the magnetic coating composition.
In order to eliminate flow orientation of the magnetic grains arising immediately after application of the magnetic coating composition, it has been proposed, for example, in Japanese Unexamined Patent Publication Nos. 53(1978)-104206 and 54(1979)-149607, to orientate the magnetic grains in one direction by a first orientation magnetic field before the applied magnetic coating composition hardens and then to orientate the magnetic grains reversely to the first orientation direction by a second orientation magnetic field of a level lower than the level of the first orientation magnetic field. However, with this method, the flow orientation cannot be eliminated sufficiently since the magnetic field can act only in the substrate movement direction and in the reverse direction.
It has also been proposed, for example, in British Pat. No. 933,762, to position a rod-like orientation magnet obliquely with respect to the movement direction of a non-magnetic substrate and to have a magnetic field act from the orientation magnet on the magnetic coating composition before it hardens, thereby randomly orientating the magnetic grains. However, with a single stage of magnetic field application, it is not always possible to achieve sufficient random orientation of the magnetic grains.